This code is modified by them in order to fully support the hardware inside the machine. Isn't it illeagal to refuse publishing a GPLed code?
Worse. It's illegal to _not_ release it together with the binary. The only allowed way to not distribute source code is including the legally binding letter as for GPL.3b. Asking politely is the best first move, though (like you did, it seems).
The point is that those resellers aren't the copyright holders on the software they distribute, so they are bound by license terms set forth by copyright holders. The same problem you have with the kernel, though, applies to every other software package they distribute but don't write; and most of them are just as GPL as the kernel is.
What they are allowed to do is distributing a customized X server and not give away source code (nor associated rights), since this is allowed by the copyright holders of the X server. The same applies for other packages with mit-style licenses, but X is still the biggest and most important such piece of code.
You are right, this is a license violation.
/alessandro